Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV)
Despite voluminous research from many different fronts, the development of safe, effective vaccines against the human immunodeficiency virus (HIV) has proven to be difficult. Like other lentiviruses, HIV has a remarkable ability to persist and to eventually induce a chronic, debilitating disease despite an apparently strong host immune response to the virus. HIV-infected humane may remain clinically well for years while maintaining detectable humoral and cellular immune responses, only to succumb eventually to the virus.
Simian immunodeficiency virus (SIV) is a nonhuman primate lentivirus that is the closest known relative of HITV-1 and HIV-2 strains. SIV closely parallels its human counterparts in genetic organization and biological properties. Similarities between SIV and HIV include: lentiviral morphology; tropism for CD4 lymphocytes and macrophages; extra genes called tat, rev, vif, vpr, and nef that other retroviruses are not known to have; use of the CD4 molecule for receptor; cytopathicity; and the ability to cause chronic disease after long-term persistent infection. All replication-competent retrovirus genomes contain gag (group-specific core antigen), pol (polymerase), and env (envelope) genes. HIV-1 has at least six additional genes: viral infectivity factor (vif), vpr, vpu, transactivator (tat), regulator of viral expression (rev) and negative effector (nef) genes; SIV from rhesus macaques (Macaca mulatta) and HIV-2 have a similar set of accessory genes as HIV-1 except that instead of vpu, the latter two have vpx. The vif, vpr, vpu, vpx, and nef genes have been termed "nonessential" since they can be deleted without completely abrogating the ability of the virus to replicate (Kestler, H. W., et al., Cell, 65:651-662 (1991)). Unique strains of SIV have been recovered from several African primate species. In the natural host of origin, these viruses establish a latent or persistent infection and appear not to cause disease. Distinct strains of SIV have also been recovered from captive Asian macaques. An SIV from rhesus macaques (SIVmac) and one from a sooty-mangabey monkey (SIVsmm) cause persistent infections in experimentally inoculated macaques, resulting in acquired immunodeficiency diseases (AIDS)-like disease that is similar to AIDS in individuals infected with HIV-1 or HIV-2, which involves immunodeficiency and opportunistic infections, and results in death {McCune, J. M., Cell, 64:351-360 (1991) and Simon, M. A., et al., AIDS Res. Hum. Retroviruses, 8:327-337 (1992)}. SIV uses the same CD4 receptor as does HIV on human T cells, and it can be blocked with the same monoclonal antibodies. Molecular clones of SIV isolates from several primate species show a genomic organization that is similar to that of HIV, and phylogenetic analysis of viral genome sequences has revealed the close evolutionary relationships of these simian and human lentiviruses {Myers, G., et al., Los Alamos National Laboratory, Los Alamos, N. Mex. (1992)}. Based on the antigenic genetic morphologic and functional similarities shared by HIV and SIV, SIV infection of macaques has come to be recognized as an animal model for HIV infection and AIDS {Hirsch, V. M., et al., Virology, 3:175-183 (1992)}. This animal model is critical for elucidating mechanisms of pathogenesis and for the development of vaccines and anti-viral therapies. The above factors, together with the close immunological relationship of primate genera, argue that a vaccine proven to be effective in protecting rhesus macaques from infection and disease after experimental challenge with SIV will likely be effective in protecting humans at risk for HIV infection and AIDS {Murphey-Corb, M., et al., Science, 246:1293-1297 (1989)}.
Two closely related molecular clones of SIVmac (SIVmac239and SIVmac1A11) have been extensively characterized iil vitro and in vivo. At the New England Regional Primate Research Center, a provirus was molecularly cloned from the SIVmac239isolate to produce a cloned virus that was also designated SIVmac239{Naidu, Y. M., et al., J. Virol, 92: 491-4696 (1988)}. This clone replicates in peripheral blood mononuclear cells (PBMCS) but is restricted in macrophages {Bancroft, A. J., et al., J. Immunol., 150:1395-402 (1993) and Ringler, D. J., et al., Lab. Inves., 62:435-43 (1990)}. SIVmac1A11, cloned at the Department of Medical Pathology, University of California, Davis, infects rhesus macaque PBMCs and both monocyto-derived and alveolar macrophages {Marthas, M. L., et al., J. Med. Primat., 18:311-319 (1989); Bancroft, A. J., et al., J. Immunol., 150:1395-402 (1993) and Unger, R. E., et al., J. Med. Primatol., 24:74-81 (1992)}. The complete sequences of the proviral clones of SIVmac1A11(GenBank accession number M76764) and SIVmac239(GenBank accession number 33262) have been determined, the genes for these viruses show greater than 95% homology {Luciw, P. A., et al., AIDS Res. Hum. Retroviruses, 8:395-402 (1992) and Regier, D. A., et al., AIDS Res. Hum. Retroviruses, 6:1221-1231 (1990)}.
Previous attempts to develop vaccines for SIV have either failed to provide immunity or had limited success. Inactivated whole-virus, virion subunits, and live recombinant subunit vaccines have all provided limited or no protective immunity against infection with virulent STV in rhesus macaques. Inactivated whole-virus vaccines have provided protective immunity to macaques against challenge with SIV propagated in human, but not rhesus, PBMCs {Carlson, J. R., et al., Aids Research and Human Retroviruses, 6:1239-1246 (1990); Desrosiers, R. C., et al., Proc. Natl. Acad. Sci. USA, 86:6353-6357 (1989); Johnson, P. R., et al., Proc. Natl Acad. Sci. USA, 99:2175-2179 (1992); Murphey-Corb, M., et al., Science, 246:1293-1297 (1989) and Gardner, M. B.; AIDS/HIV Treatment Directory, compiled and published by AmFAR, Vol. 6: 5-10 (1992)}. Evidence has been presented that the protective antigens were not viral but human cellular antigens (HLA DR, .beta.2m, and HLA class I) {Arthur, L. O., et al., Science, 258:1935-1938 (1992); Langlois, A. J., et al., Science, 255:292-293 (1992); Stott, E. J., Nature, 253;393 (1991) and Sutjipto, S., et al., J. Virol, 64:2290-7 (1990)}.
a) SIVmac1A11: In rhesus macaques infected by the intravenous route (IV), SIVmac1A11establishes a low virus load in which virus can be isolated from PBMCs during the first 2 to 6 weeks post-infection, but is not recoverable from PBMCs for observation periods of up to three years thereafter (Marthas, M. L., et al., J. Med. Primal., 18:311-319 (1989) and Marthas, M. L., et al., J. Virol., 64:3694-3700 (1990)). Macaques infected with SIVmac1A11have remained healthy over three years {Marthas, M. L., et al., J. Virol., 64:3694-3700 (1990); Marthas, M. L., et al., J. Virol., 67:6047-6055 (1993)). Because SIVmac1A11in macaques (I) caused no signs of disease, (ii) established a low virus load, and (iii) induced antiviral immune responses, its efficacy as a live attenuated vaccine was tested {Marthas, M. L., et al., J. Virol., 64:3694-3700 (1990)}. Macaques immunized with SIVmac1A11were resistant to infection with a low dose of uncloned pathogenic SIVmac251, a strain distinct from but closely related to SIVmac239, inoculated by the IV route {Marthas, M. L., et al., J. Virol., 67:6047-6055 (1993)}. However, macaques immunized with the live attenuated virus were not protected from challenge with virulent virus by the intravaginal route {Marthas, M. L., et al., J. Virol., 67:6047-6055 (1993)}.
b) SIVmac239and SIVmac239.DELTA.nef: In rhesus macaques infected by the IV route, SIVmac239persists at relatively high virus loads and causes a fatal AIDS-like disease, generally in a period ranging from 6 months to one and one-half years after injection {Kestler, H. W., et al., Science, 248:1109-1112 (1990)}. The nef gene of SIVmac239is prematurely truncated by a stop codon {Regier, D. A., et al., AIDS Res. Hum. Retroviruses, 6:1221-1231 (1990)}. The nef gene is encoded by an open reading frame located at the 3' end of the HIV and SIV genomes that extends from the end of env and partially overlaps the 3' LTR. Net encodes an early, non-structural, intracellular protein whose biochemical function and in vitro effect on virus growth are still in dispute {Kestler, H. W., et al., Cell, 65:651-662 (1991)}.
Analysis of SIVmac239recovered from infected animals revealed that a reversion in the stop codon, which truncates the nef gene, occurred in vivo to produce viral genomes encoding full-length nef genes (SIVmac239/nef-open) {Kestler, H. W., et al., Cell, 65:651-662 (1991)}. Thus, it was inferred that the nef gene is important for viral pathogenesis. Accordingly, a clone with a large non-reversible deletion of nef was constructed to produce the virus designated SIVmac239.DELTA.nef. Tests of the nef-deleted virus in rhesus macaques revealed that virus loads were low (compared to macaques infected with SIVmac239), antiviral antibodies were induced, and infected animals remained healthy for more than 3 years after infection {Daniel, M. D., et al., Science, 258:1938-1941 (1992)}.
The vaccine potential of the live-attenuated SIVmac239.DELTA.nef was tested by immunizing macaques with this virus and then challenging with virulent virus strains (i.e., cloned SIVmac239/nef-open and uncloned SIVmac251) {Daniel, M. D., et al., Science, 258:1938-1941 (1992)}. Evaluation of plasma antigenemia and virus loads in PBMCs revealed that three of four immunized animals showed no signs of infection after challenge with SIVmac239/nef-open. One of these four immunized animals demonstrated a transient infection: virus was recovered from PBMCs at four weeks after challenge. In addition, two of these macaques were protected from challenge with 1000 animal infectious doses (AID) of uncloned SIVmac251. However, SIVmac239.DELTA.nef causes persistent infection in macaques, a major limitation for its use as a vaccine {Daniel, M. D., et al., Science, 258:1938-1941 (1992)}.